One reason is the cost of the yttrium addition process coupled with the fact that appreciable quantities of yttrium have to be used to effectively reduce the available, active sulphur content in the alloy from 5-15 ppm to about 1 ppm by weight (w). Further, yttrium is itself a chemically very reactive element and will not only actively combine with sulphur but also with oxygen to form yttrium oxides and oxysulphides. These oxides (Y.sub.2 O.sub.3) and oxysulphides (Y.sub.2 O.sub.2 S) can nucleate grain defects in single crystal nickel-base alloy castings making the castings unusable and, therefore, necessitating their rejection. Further, a nickel yttrium eutectic phase can form which has a low melting point, substantially reducing the solution heat treat temperature which can be applied to the single crystal components during manufacture. This is particularly important in the case of aircraft turbine engine airfoils subject to very high temperature operating environments, up to 2100.degree. F. The restricted solution heat treat temperature results in reduced alloy strength and phase stability thus materially reducing turbine blade useful life.
This invention provides a workable solution to the problem of single crystal alloy cyclic oxidation resistance and phase stability under conditions of very high operating temperatures, for example at turbine blade tips, by substantially eliminating sulphur and at the same time materially reducing the quantity of yttrium required in the turbine blade components. It is not possible to entirely eliminate sulphur and, at the same time, it has been found to be impossible to entirely eliminate yttrium.
In an effort to develop an alloy having the desired characteristics for use in high efficiency gas turbine engines operating at high temperature, the alloy sold under the Cannon-Muskegon's trademark "CMSX-4" was considered to have the basic functional characteristics. This alloy is disclosed in U.S. Pat. No. 4,643,782, entitled "SINGLE CRYSTAL TECHNOLOGY" issued Feb. 17, 1987. This alloy has many of the characteristics which are desirable when applied to the high temperature turbine airfoils which are the objective of the improved alloy set out in this disclosure. As will be noted from Table I, the alloy of U.S. Pat. No. 4,643,782 includes, among other elements, 20 (w) ppm max. of sulphur. Also, 30-100 (w) ppm of yttrium may be included in the single crystal turbine airfoil components to appreciably improve bare alloy cyclic oxidation resistance, i.e., reduce aluminium oxide spalling, which is particularly important for the tip regions of modern, shroudless turbine blades and transpiration cooled turbine airfoils.
Sulphur has long been recognized as troublesome in this type of high temperature nickel-base alloy. Sulphur, although in small or trace amounts can be acquired by an alloy from the refractory linings or crucibles in which the alloy is melted or remelted at temperatures in the range 2700.degree. F.-2850.degree. F. To avoid this, the refractory linings in which the alloy is melted are made from costly and very pure materials. For this purpose, linings preferably made of magnesium oxide and aluminium oxide spinel-forming refractories are utilized. Vacuum induction furnace atmospheres have to be extremely clean and essentially sulphur-free.
In addition, very careful selection of raw materials used for the alloy is practiced to avoid unwanted addition of sulphur together with maintenance of ultra cleanliness of the vacuum induction furnaces and pumping systems. It should be noted that vapor booster oil contains sulphur and hence even slight back-streaming of vapor booster oil from the vacuum pumps into the furnace melting chamber or pouring chamber is not permissible. In the manufacture of the alloy, care is taken to keep sulfur at a very low level and also to maintain a very low oxide inclusion content. Extensive research and melting trials have found it possible to consistently produce CMSX-4 alloy with a sulphur content of 1 (w) ppm. This has now been done and repeated with six heats (V8256, V8276, V8277, V8291, V8311 and V8312) of 8000 lb. each with consistent reduction of sulphur from the former 4-6 (w) ppm range to a 0.8-1.7 (w) ppm range with an average of 1.0 (w) ppm. The analytical technique used for sulphur analysis is high resolution glow discharge mass spectrometry [GDMS]. It is postulated that phosphorus may play a similar deleterious role to sulphur. The phosphorus content of these heats has been reduced to a range of 0.7-1.1 (w) ppm, analyzed using GDMS.
Having, in effect, almost eliminated the sulphur problem there remains the yttrium problem. While the addition of yttrium has the dramatic effect of reducing cyclic, bare alloy oxidation almost to zero under high temperature operational conditions, yttrium has other undesirable effects upon other critical characteristics of the alloy. Yttrium forms a low melting point, eutectic phase identified as nickel yttrium which has a much reduced melting point, thus reducing the melting point for the entire alloy. Thus, the alloy's solution temperature is reduced to the point that the solution temperature necessary to enable the alloy to be fully solutioned and thus develop its important characteristics, that are, creep and fatigue strength and phase stability under sustained high temperature conditions, cannot be attained due to occurrence of unacceptable incipient melting, with attendant pore formation and excessive residual microsegregation.
Because of the high reactivity of yttrium it has heretofore been necessary to add an excess quantity of this element to obtain the results which are considered desirable in the finished casting. This, however, is not a desirable approach because yttrium is very reactive and at the elevated temperatures at which this alloy is single crystal cast, yttrium readily forms yttrium oxide inclusions from reaction with remelting ceramic crucibles, shell molds and cores which nucleate grain defects resulting in unacceptable, reject airfoil castings.